Chapter 5 The Galbulimima Alkaloids

This chapter presents information on Galbulimima alkaloids. In an earlier review of the Galbulimima alkaloids, it was noted that most of the alkaloids fell into the following three classes: (1) tetracyclic lactones, (2) highly oxygenated hexacyclic esters, and (3) one hexacyclic and two pentacyclic bases of low oxygen content. The discussion was focussed on the chemistry of himbacine (I) and the other members of class 1—namely, himbeline, himandravine, and himgravine. The ester alkaloids have structures related to that of himbosine, whereas the members of class 3 have somewhat different structures; himgaline has the structure III and himbadine, IV. Alkaloid G.B. 13 is the de-N-methyl derivative (V) of himbadine. In the elucidation of this array of structures, the major ester alkaloid himandridine occupied a central position. Once structure VI had been established for himandridine, both by rigorous chemical proof and by correlation with himbosine, it became possible to circumvent degradative approaches and to derive the structures of the other alkaloids by means of a number of correlations. First, a variety of chemical and spectroscopic correlations revealed the structures of the remaining ester alkaloids. The way then opened to effect a partial synthesis of alkaloid G.B. 13, and hence himbadine from an ester alkaloid, himandrine. The structure of himgaline was proved by the rational conversion of alkaloid G.B. 13 into himgaline derivatives.